The oligomerisation of olefins, primarily α-olefins, with chromium catalysts has been extensively studied. More specifically, a number of chromium catalysts have been developed and used to trimerise olefins. In this regard, the trimerisation of ethylene to 1-hexene is significant since, in addition to its use as a specific chemical, 1-hexene is extensively used in polymerization processes either as a monomer or co-monomer. Furthermore, the trimeric products derived from longer chain olefins could be well utilized as synthetic lubricants (e.g. polyalphaolefins/PAOs), as well as various other applications such as components of drilling muds, and as feedstock to prepare detergents and plasticizers.
Prior art chromium based ethylene trimerisation processes include:    a) A process in which olefins are trimerised by passing the olefin in contact with a catalyst comprising the reaction product of a chromium compound, an organoaluminium compound hydrolyzed with a specific amount of water and a donor ligand selected from hydrocarbyl isonitriles, amines and ethers (U.S. Pat. No. 4,668,838);    b) A process to trimerise ethylene to 1-hexene comprising contacting ethylene with a stabilized catalyst system comprising a chromium source, a pyrrole-containing compound, a metal alkyl and an aromatic compound (European Patent No. 0 668 105);    c) A process for preparing α-olefin oligomers, which comprises carrying out oligomerisation of an α-olefin in a solvent by reacting said α-olefin with a chromium-based catalyst system comprising a combination of at least a chromium compound, an amine or metal amide, and an alkylaluminium compound, in a contacting mode that the chromium compound and the alkylaluminium compound are not previously contacted with each other (U.S. Pat. No. 5,750,817);    d) A process for oligomerising ethylene to produce 1-butene and/or 1-hexene wherein catalytic composition is obtained by mixing at least one chromium compound with at least one aryloxy aluminium compound with general formula RnAl(R′O)3−n where R is a linear or branched hydrocarbyl radical containing 1 to 30 carbon atoms, R′O is an aryloxy radical containing 6 to 80 carbon atoms and n is a whole which can take the values 0,1 or 2, and with at least one other hydrocarbyl aluminium compound selected from tris(hydrocarbyl)aluminium compound or chlorinated or brominated hydrocarbyl aluminium compounds (U.S. Pat. No. 6,031,145); and    e) A process for the trimerisation of ethylene, said process comprising reacting ethylene, using a catalyst comprising an aluminoxane and a polydentate phosphine, arsenic and/or antimony coordination complex of a chromium salt, such that 1-hexene is formed (U.S. Pat. No. 5,811,618).